1. Field of the Invention
The present invention relates to an electric motor comprising a rotor part, and a sleeve mounted to a rotary shaft to hold the motor part.
2. Description of the Related Art
Japanese Unexamined Utility Model Application Publication No. 05-26202, for example, discloses, a so-called built-in motor. In this motor, a rotor is mounted to a rotary shaft via a sleeve. On an inner circumferential surface of the sleeve, a plurality of protrusions are formed which radially project toward an outer circumferential surface of the rotary shaft. The protrusions annularly extend around a rotation axis of the rotary shaft. On the other hand, the rotary shaft is formed such that its diameter varies, for example, in three steps from one end to the other end thereof. As the diameter of the rotary shaft varies, the protrusions correspondingly project in different lengths from the inner circumferential surface of the sleeve. Thus, the sleeve is formed as a so-called stepped sleeve. In mounting the rotor to the rotary shaft, the sleeve is mounted by shrinkage fit to the rotary shaft, and is pressed at the protrusions against the outer circumferential surface of the rotary shaft.
On the other hand, between the protrusions, spaces are formed between the inner circumferential surface of the sleeve and the outer circumferential surface of the rotary shaft. The electric motor needs to be constructed such that the rotor can be removed from the rotary shaft, for example, at the event of malfunction of the motor. In order to remove the rotor from the rotary shaft, working fluid is injected at a certain pressure into the aforementioned spaces. As a result of increasing the inner diameter of the sleeve due to its expansion caused by the pressure of the working fluid, a force is exerted on the sleeve in the direction of the rotation axis since the plurality of protrusions have different inner diameters due to the variation of the diameters of the rotary shaft. Thus, it is possible to remove the sleeve together with the rotor from the rotary shaft in the direction of the rotation axis.
Since the sleeve is received by the rotary shaft only at the protrusions, the contact area of the sleeve is small as compared to the inner circumferential surface of the sleeve. As a result, in order to achieve robust mounting of the sleeve, it is necessary to increase the interference of the sleeve, thereby increasing tightening torque of the sleeve in relation to the rotary shaft. However, such tightening torque is concentrated onto the protrusions at both outer ends of the sleeve. In addition, due to the increase in the interference, the sleeve has to be heated to higher temperature at the time of shrinkage fit. Therefore, local unnecessary stress is exerted to the rotary shaft from the protrusions due to difference in thermal expansion between the rotary shaft and the sleeve. Consequently, the rotary shaft will be subject to deformation or distortion, and oscillation of the rotary shaft will increasingly occur.
The present invention has been made in view of the above-described problem, and thus, the object of the present invention is to provide an electric motor in which a sleeve can be mounted to a rotary shaft with high precision.